For some reason, I got a weird error on travis when running against 1.9.2 for HTTPClient, where the response was already the body string rather than an object that responds to #content. This fixes that as well.

VCR as a whole is not designed to be threadsafe (running your tests in multiple threads is asking for trouble), but the rack middleware needs to be, since some rack servers (like WEBrick) are multithreaded.
Closes#58.

- Rename it to :ignore_cassettes--I think this is more intention-revealing.
- Ensure `insert_cassette` does not actually insert a cassette when VCR is turned off with :ignore_cassettes => true.
- No need for `use_cassette` to handle this since `insert_cassette` handles it properly now.
- Add additional specs.
- Fix cassette insertion error message so it mentions the new
:ignore_cassettes option.